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FILLING BY CLASSES AND MODIFICATIONS. 223

of the greater difficulty of approaching, inspecting and
operating in the cavity. The same general principles
are applicable to the opening of this class of cavities,
as to that of the third class, except that pressure can-
not be as frequently employed, since the cavity will
not be as well exposed by this as by cutting, nor ren-
dered so easy of approach. Indeed, in operating on
these cavities the use of the mirror is often necessary,
it being impossible to obtain a direct view into them,

after having cut away all that it is admissible to re-
move. This difficulty is almost wholly obviated by
the method of working through the masticating sur-
face of the tooth into the cavity of decay, as already
described. The accessibility of these cavities will
depend on circumstances; such as the location of the

tooth, its inclination, the size of the mouth, the flexi-
bility of the muscles, and the ability of the patient to
open the mouth and keep it open. Generally, in ope-
rating on these cavities, for the removal of decay, for
the formation, and the filling, curved instruments will
be required ; but their curvature should not be greater
than the necessity of the case demands.
The cavity, during its preparation, must be fre-
quently examined. Its general form, and the size,
kind, and location of. the retaining-points will be the
same as in class third. The lateral walls, if the cavity
is not too large, should be made parallel with each